An unsolved mystery surrounding the assassination of Abraham Lincoln prompted the idea for “The Fifth Assassin” (Grand Central, $27.99). Brad Meltzer, the best-selling crime novelist and host of the History Channel’s “Decoded,” spoke about his new thriller by phone from his home in Florida.

How did you come up with the premise for “The Fifth Assassin”?

Everyone knows about when Abraham Lincoln was killed. They can tell you what John Wilkes Booth had in his wallet, and what kind of drink he had at the bar next door to Ford’s Theatre. The only detail not known to history is how Booth got past the White House valet. He very famously handed the valet a card. To this day, no one knows what was on that card. Some people say it was a calling card; some say it was a playing card, which once were used as calling cards. I loved that idea that there was this secret, mysterious card that somehow altered the history of the United States. When I went to the Museum of Health and Medicine to do research, they handed me a little leather square and said, “It’s the tattoo of Charles Guiteau, President Garfield’s assassin.” And I realized that what I was holding was not leather: It was someone’s skin. And in the corner of the tattoo was a red diamond. My brain immediately went to this mysterious card that John Wilkes Booth famously had. As a thriller writer, I started finding playing card connections: What if all these assassins were somehow working together, were part of a larger plan? Finally, I had my link.

Are you a conspiracy theorist?

I do not believe the government sits around and lies to us. People lie. And they tend to lie for very human and self-protective reasons. History, to me, is a giant game of telephone. Our job is to find the first whisper. People want to see conspiracies, but the scariest story of all is the true story.

Are the presidential assassinations depicted with historical accuracy in your novel?

I spent two years doing the research. I surrounded myself with presidential death. I’m like the Harold and Maude of 1600 Pennsylvania Ave. When you see Leon Czolgosz folding his handkerchief over and over on his hand to cover the gun as he goes to shake President McKinley’s hand, that’s real. The nice part about writing a thriller about the four men who have killed presidents is that you don’t have to come up with the crazy. They are so crazy on their own.

Of the many artifacts you combed through for this story, which was your favorite?

One of my favorites — I actually pulled it from the book — is a letter from a little girl to Abraham Lincoln telling him that if he grew a beard, he’d win the presidency. And he wound up growing the beard, and he wins. The National Archives has a letter she then writes to him asking for a job. And he basically says, “No.” And I thought: “My gosh, man, the girl gave you the beard. Give her a job!”

How much did writing comic books influence your writing of political and legal thrillers?

Whether it’s George Washington or Superman or Abraham Lincoln, to me they are all part of American mythology. Their stories persist because they reveal something about us.

Your characters are very meticulous, and a little nerdy — especially the fictional archivist Beecher White, who also was featured in your previous novel, “The Inner Circle.” Are they like you?

You officially just called me a nerd. Hey, I can’t help it! I’m a guy who spent four years studying historical details about the deaths of presidents. Of course I’m a nerd. I’m obsessed with history and always have been. I write comic books for a living! I out-nerd anyone in that alone.

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